I. Field of the Invention
The present invention relates to wireless communication systems. More particularly, the present invention relates to a novel and improved system and method for estimating, and correcting for, clock error in a remote communication device.
II. Description of the Related Art
To increase system capacity, some mobile telephone systems use so-called "slotted paging", which relies on what might be thought of as temporal multiplexing of one or more paging channels. In other words, a mobile telephone, in a slotted paging environment, is assigned periodic windows (referred to as "slots") during which the mobile telephone may be paged, with the periods between the windows being reserved for paging other mobile telephones via the same paging channel. Slotted paging is described in more detail in, for example, Telecommunications Industry Association (TIA)/Electronic Industries Association (EIA) Interim Standard IS-95 entitled "Mobile Station--Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular System."
To advantageously conserve battery power, the mobile telephone ordinarily is "powered down" between slots. By "powered down" is meant that most, but not all, of the electrical components of the mobile telephone are de-energized. When powered down, the mobile telephone is referred to as being "asleep".
One component of the mobile telephone that remains energized while the telephone is asleep is the telephone's internal clock, typically a voltage controlled temperature controlled crystal oscillator (VCTCXO). Among other functions, the internal clock meters the sleep period so that the remaining components of the telephone can be re-energized at the correct time to maintain synchronization with the paging channel and monitor for pages during the next successive slot.
It is important that the internal time of the mobile telephone is synchronized with the mobile telephone system time. This is particularly so in systems that use code division multiple access (CDMA) principles to discriminate one mobile telephone signal from another. Accordingly, during its slots in which the mobile telephone is in contact with a system base station, the base station transmits system time to the mobile telephone to facilitate synchronization.
During its sleep periods, however, the mobile telephone must rely on its internal clock to maintain time, because the mobile telephone is not in communication with the base station. Unfortunately, all clocks, including VCTCXOs;, are subject to error. For example, in the case of any voltage controlled crystal oscillator, the input voltage can vary despite efforts to maintain it at a constant operating voltage, and even small input voltage variations introduce time keeping errors. Consequently, in the case of mobile telephones, the internal clock time might not be the same as the system time when the mobile telephone "awakens", thereby undesirably delaying acquisition by the mobile telephone of the paging channel.
This error problem is complicated by the fact that the output characteristics of a VCTCXO change with temperature and with the age of the VCTCXO. As recognized herein, however, it is possible to dynamically compensate for the error of a VCTCXO in a mobile telephone, thereby promoting relatively rapid synchronization with system time when the mobile telephone "awakens".
Accordingly, it is an object of the present invention to provide a system for compensating for internal clock error during periods in which an associated mobile telephone is asleep. Another object of the present invention is to provide a system for compensating for internal clock error in a mobile telephone which dynamically determines the amount of error compensation required. Still another object of the present invention is to provide a system for compensating for internal clock error in a mobile telephone that is easy to use and cost-effective to manufacture and implement.